This application is a U.S. National Phase Application of PCT international application PCT/JP99/01281.
The present invention relates to a rotation angle detector for detecting a rotational position of a steering wheel of a vehicle.
In general, steering for vehicles such as an automobile is provided with a lock-to-lock latitude for a steering wheel between a rightmost and a leftmost positions, when a steering wheel is turned clockwise and counterclockwise for several times (normally, 3 to 6 revolutions).
A reason of the above is to reduce a physical force required for an operator to turn the steering wheel by decreasing a speed of rotary movement of the steering wheel with a steering gear. Another reason is to reduce vibrations and impacts of wheels from being transmitted to the steering wheel. It is also for a reason to improve maneuverability by increasing an amount of turning the steering wheel (i.e. steering angle) necessary for changing a heading direction of the vehicle. In other words, the vehicle would require the operator to pay an excessive degree of attentiveness, if a small turning of the steering wheel should cause a substantial change in the moving direction. Stable traveling of the vehicle is thus achieved by providing a handling characteristic broad enough and suitable for a general sense of operators.
For the same reason, the operator is unable to determine a rotational position of the steering wheel at a glance, nor is she/he able to find a moving direction of the vehicle. A rotation angle detector for detecting a position and an angle of rotation of the steering wheel has hence been suggested. That is, an operator can easily determine a moving direction of her/his vehicle (especially, whether or not it heads straight) by using the detector. The detector is also adaptable for a variety of control devices such as (1) a device that changes firmness of a suspension according to an angle of a steering wheel, (2) a device for alarming an operator in case a steering wheel is turned beyond a predetermined angle, and (3) a device for guiding the operator for a way to return home.
FIG. 5 shows a structure of a rotation angle detector of the prior art. In the figure, a reference numeral 1 represents a disc having a first series of through holes 1a provided thereon with a predetermined pitch, and it is fixed to a rotor 2. Reference numerals 3a and 3b respectively represent a first and a second sensor elements, each comprising a transmission type photo-interrupter consisting of a light-emitting element and a light-receiving element positioned in a manner to face each other across the first series of through holes 1a. The two sensor elements are provided side by side in order to detect a direction of rotation. The first sensor element 3a and the second sensor element 3b are prearranged to form a predetermined angle between them. The disc 1 also has a cutout hole 1b, and a third sensor element 4 comprising another transmission type photo-interrupter is provided for detecting the cutout hole 1b, so that the detector detects a reference position for rotation of the rotor 2 by the third sensor element 4 with the cutout hole 1b. 
The rotor 2, the sensor elements 3a and 3b, and the sensor element 4 are all housed in a case 5. The rotor 2 is so arranged that it rotates freely in synchronization with a rotation of the steering wheel in a vehicle or the like.
A mutual relation and operation of the foregoing structural elements of the prior art detector will be described hereinafter. When the steering wheel is made in a vehicle, it rotates the rotor 2 in a link motion, as well as the disc 1 fixed to the rotor 2, and the sensor elements 3a and 3b detect a number of the through holes 1a in the disc 1. The detector is able to measure an angle of the rotation of the steering wheel and to detect a direction of the rotation, using the number of the through holes 1a detected by the first and second sensor elements 3a and 3b as a coefficient, since the through holes 1a are arranged with a predetermined angle pitch, and the first and second sensor elements 3a and 3b are disposed with a difference of 90 degrees in electrical phase between them.
On the other hand, the sensor element 4 can produce a reference position signal, when the cutout hole 1b passes through it as the rotor 2 and the disc 1 rotate. Thus, the detector is able to determine a present angle of the rotation of the steering wheel by calculating an amount of change in angle of rotation of the steering wheel obtained as above, with reference to a position where the reference position signal is obtained by the sensor element 4 from the cutout hole 1b. 
Accordingly, the angle and direction of the rotation of the steering wheel can be calculated by processing the number and phase of detected signals for the angle and direction of the rotation with an electronic circuit. Subsequently, a rotational position of the steering wheel can be determined from the angle and direction of the rotation of the steering wheel.
When an engine of the vehicle is not running (an ignition key is in an xe2x80x9cOFFxe2x80x9d or an xe2x80x9cAccessoryxe2x80x9d position), electric power is not supplied to the rotation angle detector. The detector is unable to determine an angle of the rotation of the steering wheel until the cutout hole 1b passes through the sensor element 4 after the engine is started. Also, the detector is unable to determine how many revolutions the steering wheel has been turned at any point of time from only passage of the through holes 1a by the sensor elements 3a and 3b, since the steering wheel is made to turn complicatedly. In general, therefore, the detector is designed to determine a neutral position of the steering wheel by making a comparison of the passage data of the through holes 1a with information of number of rotations between right and left wheels, etc. In other words, the detector is unable to make a conclusive determination of an angle of rotation of the steering wheel until the vehicle makes a straight travel after the engine is started.
In order to solve the above problem, a method has been employed in some of the devices in that an electronic circuit is equipped with a memory element, which functions even when the engine is in a state of standstill, for storing information of a rotational position of the steering wheel immediately before the engine is turned off.
However, an inconsistency occurs between an actual angle of rotation and the stored angle of rotation of the steering wheel, if the steering wheel is turned while the engine is not running. This causes the detector to execute a control based on an incorrect angle of rotation of the steering wheel, and thereby giving rise to a problem that the detector is not able to carry out the intended control.
The present invention aims at solving the above-described problems, and to provide a rotation angle detector, which operate in a manner to: (1) back up memory of an angle and a direction of the rotation of a steering wheel in a memory element of an electronic circuit; (2) activate the detector by itself with own sensor element on a standby mode, when the steering wheel is turned while an engine is at a standstill; and (3) detect an angle and a direction of the rotation of the steering wheel, even when the engine is not running.
In order to solve the above problems, a rotation angle detector of the present invention comprises: (1) a disc having a plurality of through holes arranged with a predetermined pitch along a perimeteric direction, and a cutout portion provided in a predetermined position along the perimeteric direction; (2) a first and a second sensor elements, each comprising a light-emitting element and a light-receiving element, and disposed within a predetermined angle in a manner to correspond with the through holes in the disc; (3) a third sensor element comprising a light-emitting element and a light-receiving element, and disposed in a manner to correspond with the cutout portion in the disc; and (4) a control means comprising a signal processing unit for generating an output of detection signal corresponding to an angle and a direction of rotation of the disc using signals provided by the first, second and third sensor elements, and a memory unit for storing, as a positional information, the output provided by the signal processing unit, when a main switch is turned on. The control means includes: (a) a first means for introducing a dark current mode for maintaining the sensor elements operative with a lower supply current than that consumed under a normal operating condition, even when the main switch is in an OFF position; (b) a second means for introducing a memory backup mode for maintaining a storing condition of the positional information supplied from the signal processing unit by providing a current supply to the memory unit, even when the main switch is in the OFF position; (c) a third means for canceling the memory backup mode and the dark current mode so as to turn the sensor elements and the control means into their normal operating conditions, when the sensor elements operated under the dark current mode of the first means detect a motion of the disc; and (d) a fourth means for actuating the first means and the second means into operation so as to resume the memory backup mode and the dark current mode, if the sensor elements no longer detect motion of the disc while in a condition after the third means has canceled these modes.
With the foregoing structure, the detector is capable of obtaining information of an absolute steering angle immediately after the engine is started, since the detector keeps in memory a steering wheel position, monitors any movement of the steering wheel at all times, and renews the stored information if any change takes place while the engine is not running.